1. Field of the Invention
The invention relates to a system for reading coded characters which are formed on a document by a succession of groups of segments which are separated from one another by intervals of lengths selected from a predetermined set of several lengths.
The object of the invention is to provide a system capable of automatically correcting certain reading errors resulting in particular from defects in the document, which make it difficult to read by electronic means. The invention is applicable in particular but not solely to the reading of characters printed in the code referred to by the abbreviation CMC7 (coded magnetic characters composed of 7 elements) which is widely used in Europe by banks, financial institutions and commercial concerns in order to record on their checks the priniciple items of information, such as the code number of the issuing branch, that of the account of the drawer, etc.
2. Description of the Prior Art
On certain documents, special characters are formed (in particular are printed in a special ink such as a magnetizable ink) so that they can be read automatically by electronic means. Thus, current use is made of characters formed by a succession of groups of segments which are separated from one another by intervals of lengths selected from a set of n predetermined lengths. In addition, each character defined by a constant number in intervals selected from the n length is separated from its neighbor by an interval longer than the longest of the n intervals.
As mentioned above, the most widely used of these codes is the CMC7 code. This code is capable of representing alphanumeric characters which have the advantage of being readable both by machine (by the detection of a magnetic field) and by an individual, since each alphanumeric character is able to appear on the document in a slightly "chopped up" but nevertheless perfectly intelligible form. The larger the number n, the greater the number of different characters which can be coded. Thus, in the case of the CMC7 code, i.e. specifically a numerical code restricted to 15 characters (10 numerals and 5 special symbols) employed in processing checks, there are 7 segments which define 6 intervals among which there must be two long intervals and 4 short intervals. The various possible combinations of the orders in which the long and short intervals follow one another enable the characters to be differentiated. Thus, in the CMC7 code, the numeral 0 is defined by reading from left to right, two short intervals, two long intervals and two short intervals; the numeral 1 is defined by one long interval, three short intervals, one long interval and one short interval; the numeral 2 is defined by one short interval, two long intervals and three short intervals; etc. The standard sizes of the intervals are defined as follows:
center-to-center distance between two segments separated by a short distance (short interval)
a=300M
center-to-center distance between two segments separated by a long distance (long interval)
b=500M
width of a segment
1=150M
size of the very long interval (between two successive characters)
c.gtoreq.930F
In the majority of cazses, the segments, separated by the abov e-mentioned intervals, are printed in a magnetizable ink.
Various methods and apparatus for reading these coded characters are known. My prior U.S. application Ser. No. 936,693, now U.S. Pat. No. 4,182,481, discloses a bar code reading device suitable for use in the present invention and the subject matter thereof is hereby incorporated by reference.
One known reading method consists in passing a document in front of a single reading head and measuring the intervals of time between the passages of two successive segments in front of the gap of the reading head. The system suffers from the major disadvantage that it requires the speed of movement to be perfectly steady.
A second method of reading, which is described in particular in French Pat. Specification No. 2,289,010 entitled "Arrangement For Reading An Item of Information", enables the short and long intervals to be distinguished by using two reading heads (or a double reading head with two sensors) which are spaced apart by a distance equal to one of the intervals. Thus, to distinguish a long interval from a short interval, one detects whether or not the segments pass in front of the two heads simultaneously. Also, to enable the speed of movement to be reduced without incurring any disadvantages, the arrangement employs sensors which are produced from magnetoresistors which are sensitive to the strength of the received magnetic field rather than to its variation. Magnetoresistive readers are described in U.S. Pat. Application Ser. Nos. 899,383 and 899,217 which were filed on behalf the assignee of the present invention. However, in the case of the CMC7 code, it is not possible to recognise the very long interval with two magnetoresistors.
A third method of reading is described in particular in a U.S. Pat. application Ser. No. 968,789 which was also filed on behalf of the assignee of the presenst invention. It consists in using (in the case of the CMC7 code) two magnetoresistors separated by a distance e (center-to-center spacing) which meets the dual condition of inequality: EQU b-1&lt;e=a+1
The principle advantage of this third method of reading over the second method is that it enables the very long interval to be distinguished using only two magnetoresistive sensors separated by the distance e. Still in the context of the CMC7 code, if the sensor which is so disposed as to be the first to detect the presence of a magnetic segment during the relative reading motion between the document and the said sensors is called "first sensor" and the sensor which is so situated as to be the second to read the same item of information is called "the second sensor", the rule governing the recognition of the different intervals is as follows:
(1) a long interval is detected when a rising edge of a pulse generated by the first sensor is registered while a pulse generated by the second sensor is present,
(2) a short interval is detected when a falling edge of a pulse generated by the first sensor is registered while a pulse generated by the second sensor is present,
(3) a very long interval is detected when no pulse is generated by the first sensor during the time a pulse generated by the second sensor is present.
The above mentioned Patent Application Ser. No. 968,789 also shows how the principle may be applied in general to distinguishing between a larger number of intervals using a larger number of magnetoresistive sensors.
However, inter alia, for all the three methods of reading referred to above, reading errors are possible. These are mainly due either to imperfections in the printing of the characters (inking faults or the presence of tiny blemishes which may be interpreted as additional segments by the reading system) or to damage to the carrier which has caused such faults to occur in the printing of the characters. It should not be forgotten that a check is often crumpled or at least folded when it is presented for payment.